Reduced electron/hole recombination in Z-scheme nanostructure of zeolitic imidazolate framework-11/graphitic carbon nitride as photocatalyst under visible light

In this research, for the first time, the synthesis of nanostructure of zeolitic imidazolate framework-11/graphitic carbon nitride (ZIF-11/g-C3N4 X) with different weight of g-C3N4 (X: 0.01, 0.1, 0.3 g) is reported. Their performance was compared in photocatalytic degradation of MB under visible light. Synthetic samples were characterized by X-Ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectrometer (XPS), diffused reflectance spectroscopy (DRS), Field emission scanning electron microscopy (FE-SEM), Transmission electron microscope (TEM), Brunauer–Emmett–Teller (BET), Electrochemical Impedance Spectroscopy (EIS) and Photoluminescence (PL) analysis. Based on the results, Z-scheme ZIF-11/g-C3N4 0.3 was selected as the best sample. FESEM and TEM images indicated that g-C3N4 sheets were complicated on the surface of ZIF-11 with rhombic dodecahedron (RHO) morphology. The surface area and band gap of ZIF-11/g-C3N4 0.3 was determined as 174.5 m2/g and 2.58 eV, respectively. The recombination of charge carriers in the ZIF-11/g-C3N4 0.3 nanostructure was reduced. Photocatalytic degradation efficiency of MB (5 ppm), pH = 7, visible irradiation (120 W-60 min) using 0.1 g of ZIF-11/g-C3N4 0.3 was achieved 72.7% with first-order kinetic model and acceptable stability in three consecutive cycles. Further, the total organic carbon (TOC) removal rate by ZIF-11/g-C3N4 0.3 after 5 h were 66.5%.


Experimental Materials
), Ammonium Hydroxide (NH 4 OH), Hydrochloric Acid (HCl), Sodium Hydroxide (NaOH), Sodium Chloride (NaCl), Methanol (CH 3 OH) and Methylene Blue (C 16 H 18 ClN 3 S) were purchased from Merck company.All chemicals used in this work were of analytical grade, obtained from commercial supplier, and used without further purification.

Synthesis of zeolitic-11 imidazolate framework
ZIF-11 micro particle was synthesized based on our previous research with slight modifications 14 .First, 0.12 g of benzimidazole was dissolved in 4.8 g of methanol, along with 4.6 g of toluene and 1.2 g of ammonium hydroxide under continuous stirring.Then, 0.11 g of zinc acetate was added and was continuously stirred at ambient temperature for 3 h.After that, the solid was separated and washed three times with methanol to completely remove the toluene.Finally, it was dried at room temperature for 12 h.The synthetic sample was named as ZIF-11.

Synthesis of graphitic carbon nitride
The g-C 3 N 4 powder was prepared by thermal polymerization similar to our previous research work 2 .First, 16 g of urea was heated in a 100 mL aluminum crucible with a lid for 4 h at 550 °C by 2 °C/min.Finally, a yellow powder product has been obtained.The synthetic sample was named as g-C 3 N 4 .

Synthesis of zeolitic-11 imidazolate framework/graphitic carbon nitride
Zeolitic imidazolate framework-11/graphitic carbon nitride was synthesized by a simple method at room temperature.First, a certain amount of synthesized g-C 3 N 4 was dispersed in 6.1 mL methanol for 150 min (solution A).Then, another solution was prepared by dissolving 0.12 g of benzimidazole in 6.1 mL methanol, 5.3 mL toluene and 0.8 mL ammonia and finally adding 0.11 g of zinc acetate (solution B).Afterward, solution A was added to solution B and stirred for 3 h at room temperature.Then, the solid was separated and washed three times with methanol.Finally, it was dried at room temperature for 3 h (Fig. 1a).The synthetic samples were named as ZIF-11/g-C 3 N 4 X.The value of X was the weight of g-C 3 N 4 (0.01, 0.1, and 0.3 g) which was used in the synthesis.

Characterization
The crystal structure of the material was determined by X-ray diffraction (D8-Advance, Bruker, Germany) using Cu Kα radiation (λ = 1.5406Å) in the 2θ range from 10° to 80°.Fourier transform infrared spectroscopy (FTIR, SHIMADZU 8400S, Japan) has been used to study the structure and chemical bonds of the molecule.X-ray photoelectron spectrometer (XPS) with a monochromated AlKα source at a power of 180 W (Specs/ FlexPS, Germany).The surface morphology of the sample was observed with a field emission scanning electron microscope (FESEM, ZISS).Energy dispersive X-ray spectroscopy (EDS) (MIRA III SAMX detector, France) was performed to determine the percentage of elements in the composition.The transmission electron microscope (TEM) image was obtained from a Phillips model EM208S device.Nitrogen adsorption-desorption isotherms (BELSORP MINI II, BEL company) were recorded with Quantachrome at liquid N 2 temperature.The specific surface area was determined from the linear part of the BET diagram.The light absorption properties of solid samples were determined by UV-visible DRS (Avaspec-2048-TEC, Netherlands).In order to investigate the degree of electron/hole separation, photoluminescence (PL) spectrum (Avaspec-2048-TEC, Netherlands) and electrochemical impedance (EIS) (Sharif solar co, model PGE-18) were determined. www.nature.com/scientificreports/

Photocatalytic performance
The photocatalytic degradation of MB in an aqueous medium was investigated in a homemade Pyrex photoreactor in batch mode (Fig. 1b).
In this way, the photocatalytic activity of the synthetic samples was investigated using a 200 mL of MB solution (5 ppm) using 0.1 g of synthesized photocatalyst under visible light irradiation (120 W).For this purpose, MB must reach the adsorption/desorption equilibrium on the surface of the photocatalyst, so the catalyst was exposed to dark for 60 min, then lamps was turned on and after 60 min of irradiation, the photocatalyst was separated from the solution by centrifugation.The efficiency of photocatalytic degradation of MB (D %) by the synthesized samples was evaluated by measuring the decrease in the intensity of the absorption band (%D = C 0 −C C 0 ) at the wavelength of 664 nm by UV-Vis spectrophotometer.The concentration of MB at illumination time t was recorded as C.

Optimization of photocatalytic degradation of MB
In order to investigating the effective factors on the process, optimization and interaction between them, the design of the experiment was carried out.Response surface methodology (RSM) is one of the mathematical and statistical methods, a common method of which is central composite design (CCD), to build experimental models of the studied process 10,11,24 .Three operational parameters of the degradation process (photocatalyst concentration, initial pH value of the solution and irradiation time) were optimized by the RSM method.The photocatalytic degradation efficiency of MB (with a constant concentration of 10 ppm in all statistical experiments) was selected as the response.By performing preliminary tests, the levels of selected factors were determined and listed in Table 1.Calculations related to the design of experiments were performed by Design Expert 11 software.

Characterization
Crystal structure and phase purity of synthesized samples (g-C 3 N 4 , ZIF-11, ZIF-11/g-C 3 N 4 0.01, ZIF-11/g-C 3 N 4 0.1 and ZIF-11/g-C 3 N 4 0.3) was studied by XRD and shown in Fig. 2a.According to the XRD pattern of g-C 3 N 4 sample, it has two characteristic diffraction peaks at 27.3° and 13.4° corresponding to (200) and (001) planes, which confirms the crystal structure (JCPDS No. 87-1526) 2,21 .In the ZIF-11 sample, the sharp peak had appeared at 4.3° belonging to the (110) plane (CCDC No. 602545) 14 .The characteristic peaks of ZIF-11 and g-C 3 N 4 have also appeared in all composites.In the ZIF-11/g-C 3 N 4 0.01 sample, the peak of g-C 3 N 4 index was not observed due to its low content in the structure, and with the increase in the amount of g-C 3 N 4 , the intensity of its index peak also increased.The crystallite size using the Debye-Scherer equation for g-C 3 N 4 , ZIF-11, ZIF-11/g-C 3 N 4 0.1, ZIF-11/g-C 3 N 4 0.3 were calculated as 20, 26.9, 26.5, 22.4 and 22.4 nm, respectively.
The FTIR spectra of three samples g-C 3 N 4 , ZIF-11 and ZIF-11/g-C 3 N 4 0.3 have been investigated using FTIR spectroscopy (Fig. 2b).According to spectrum of ZIF-11, the peak that occurs at 421 cm −1 was related to Zn-N stretching.Peaks at 1471 and 1610 cm −1 were attributed to C-C stretching in the aromatic benzimidazole ring, and peaks in the region of 600-1500 cm −1 were related to total stretching or bending of the benzimidazole ring 14,25 .Three main regions for g-C 3 N 4 can be seen.A sharp peak corresponds to 3-S triazine units was observed  www.nature.com/scientificreports/ The morphology of all synthesized samples was observed by FESEM (Fig. 4a-e), which shows that the assynthesized ZIF-11 samples have a dodecahedron structure 14,25 while g-C 3 N 4 shows a thin sheet-like structure 2 .The images of ZIF-11/g-C 3 N 4 composites (Fig. 4c-e) show the RHO morphology of ZIF-11 on the rough surface of g-C 3 N 4 .It was found that the g-C 3 N 4 sheets surrounded ZIF-11 crystals from different directions and, of course, kept the crystal structure of ZIF-11.Next, in Fig. 4f, TEM was also used further ZIF-11/g-C 3 N 4 0.3 morphology.As shown in Fig. 4f, ZIF-11 reveals a RHO assembly whereas g-C 3 N 4 shows a thin sheet-like structure which can be attached to the ZIF-11 surface.Combined with SEM, TEM results prove that g-C 3 N 4 successfully attached to the ZIF-11.The EDS analysis (Fig. 4g-k) confirmed the correctness of the synthesis of pure samples.
N 2 adsorption-desorption isotherm was investigated in order to obtain more information about the specific BET surface areas and the pore structure of the samples.The results are shown in Fig. 5. Also, the specific surface area, pore volume and pore size data of samples are listed in Table 2.As shown, the ZIF-11/g C 3 N 4 0.3 sample shows a higher specific surface area than pure g-C 3 N 4 , which is due to the contribution of ZIF-11, which has a large specific surface area in the structure.Increasing the surface area by introducing of MOF was reported in previous study 26 .All samples have pore diameters in a range of 2-50 nm, therefore, they have mesoporous structure.

Photocatalytic degradation of MB under visible light irradiation
The photocatalytic degradation efficiency of MB (MB: 5 ppm, pH: 7, Photocatalyst: 0.1 g, 120 W visible light irradiation, time: 60 min) was investigated and compared by ZIF-11, g-C 3 N 4 , ZIF-11/g-C 3 N 4 0.01, ZIF-11/g-C 3 N 4 0.1 and ZIF-11/g-C 3 N 4 0.3, which was obtained 16.7, 25.9, 70.6, 72.7 and 51.2%, respectively.Among composites, ZIF-11/ g-C 3 N 4 0.01 has the lowest degradation efficiency and ZIF-11/g-C 3 N 4 0.3 sample has the highest, and all of them have shown better performance than pure ZIF-11 and g-C 3 N 4 .The synergistic effect of the two components in improving the ability to absorb visible light, as well as in accordance with the PL spectrum, the effective charge transfer between the two components and subsequently the reduction of electron/hole recombination has led to higher destruction efficiency.Based on the obtained results, the ZIF-11/g-C 3 N 4 0.3 sample was selected as the best synthetic sample and in following, optimization of degradation was done by using it.
With the aim of investigating the effective factors on the process, the interactions between them and their optimization, an experimental design based on CCD for the photocatalytic degradation of MB was carried out using ZIF-11/g-C 3 N 4 0.3 sample.The results of the design are presented in Table 3.
The obtained data was in good agreement with the reduced quadratic model.The obtained model for predicting the degradation percentage in the coded form is: The adequacy of the obtained model was investigated with analysis of variance (Table 4).The lack of fit which compares the error of the residuals with the net error was checked and its p-value was equal to 0.0001.Due to the fact that the appropriate p-value for this parameter should be less than 0.05, the value obtained from the current working model is within the acceptable range.Also, the regression coefficient of the model showed that 94.44% of the MB degradation variable can be obtained by the model and its magnitude is sufficient to prove the adequacy and importance of the model.
According to the results obtained in the designed model, there are two binary interactions.Regarding the binary interaction of time and photocatalyst concentration, the degradation rate increases with two factors, simultaneously.By increasing the photocatalyst concentration, the surface area and number of active surface sites were increased.And also, with more time for the degradation of pollutants, the degradation rate increases.In a similar way, in the studies of Pinheiro et al., the interaction effect of time and amount of CeO 2 /g-C 3 N 4 / Ag catalyst has been investigated simultaneously.According to their results, with the increase of photocatalyst concentration and time simultaneously, the degradation rate has been increased 18 .
In order to investigate the interaction effect of pH and time on the MB degradation, these two factors were investigated (Fig. 7).Assuming that other conditions are constant, with the increase in pH, the amount of pollutant adsorbed on the active surface of the photocatalyst also increases and the degradation rate increases, but there is no significant difference in the degradation rate with the change of time.In other words, pH has been a more effective factor than time in our work.Photocatalytic removal of pollutants includes two main processes: adsorption and photocatalytic oxidation.The mechanism of photocatalytic oxidation may be described by a sequence of primary reaction steps: excitation, recombination, charge trapping, generation of oxidizing species, (1) and organic decomposition.Adsorption includes the adsorption of reactants and water.In an aqueous medium, the surface of ZIF-11/g-C 3 N 4 0.3 is easily hydroxylated.Cations can bond to the oxygen atoms of water molecules that are adsorbed on the surface of the photocatalyst and decompose into OH groups.In other words, pH plays the main role in the adsorption of the reactant on the surface of the photocatalyst 29 .The effect of pH on adsorption can be explained based on the point of zero charge (pzc), where pH pzc = 6.75 was obtained for ZIF-11/g-C 3 N 4 0.3.At lower pH than pH pzc , the photocatalyst surface has a positive charge, and at higher pH, the photocatalyst surface has a negative charge.In the case of the binary interaction of pH and photocatalyst concentration, as can be seen in the diagram, these two factors have a significant direct effect, in such a way that with the increase of each of these factors, the degradation rate increases, and with the increase of both simultaneously, higher rate of destruction can be achieved and vice versa.The reason for the higher performance of the photocatalyst in alkaline pH is the electrostatic attraction between the surface of the negatively charged photocatalyst and the cationic MB dye.Also, the increase in the concentration of OH − is a factor in the increase of OH• radicals, as a result, destruction is better and the amount of pollutant is placed on the surface of the catalyst.On the other hand, with the increase in the concentration of the photocatalyst, the surface area increases and the number of active sites is more.In the studies of Fathi et al., the results obtained in the binary interaction of pH and concentration of photocatalyst (MgO/g-C 3 N 4 /zeolite) have witnessed an increase in the degradation rate by increasing both at the same time 19 .
The optimization of the operating parameters was defined by choosing the minimum irradiation time, the minimum amount of photocatalyst and the maximum achievable degradation efficiency.The conditions are presented in Table 5. UV-Vis absorption spectra of MB using ZIF-11/g-C 3 N 4 0.3 in optimum operating condition was shown in Fig. 8.
In the following, the kinetics of the degradation reaction and the stability of the photocatalyst (ZIF-11/g-C 3 N 4 0.3), as well as the mechanism of charge transfer in the synthetic photocatalyst have been investigated in optimized condition (according to Table 5).
The stability of ZIF-11/g-C 3 N 4 0.3 photocatalyst was evaluated by repeating MB degradation experiments under visible light irradiation under the same conditions in three consecutive cycles.As shown in Fig. 9a, the photocatalytic degradation efficiency is 77.1, 68.5 and 67.3%, respectively.Photocatalytic degradation efficiency has decreased slightly after 3 cycles, which shows the stability of ZIF-11/g-C 3 N 4 0.3 sample.According to Fig. 9b, ZIF-11/g-C 3 N 4 0.3 was shown the highest mineralization capability, by which 67.5% of TOC was mineralized after 5 h irradiation, indicating that most of MB as well as the organic intermediates were mineralized and decomposed.www.nature.com/scientificreports/ The effect of pH value on the reaction rate was investigated in three experiments by choosing pH = 7, 10, 12 and the results are shown in Fig. 9c.The kinetics of the reactions follow the pseudo-first-order model, and their rate constants are determined 0.0019, 0.008 and 0.0374 min −1 , respectively.The results indicate that the rate of the degradation reaction is higher at alkaline pH values.As mentioned above, with the increase in pH, the concentration of hydroxide anions gradually increases, the conditions for the production of hydroxyl oxidizing radical are provided and it causes more destruction of pollutant 19,29 .As a result, it has caused an increase in the rate of degradation.Also, in the case of the FTIR spectrum, XRD pattern, XPS spectra and TEM images obtained after the photocatalysis of MB (Fig. 10), there is no change of the positions of bands and morphologies of composite.By XPS analysis, no obvious change regarding ionic state of Zn, C and N, and chemical composition on the photocatalyst surface was detected.These results confirmed the chemical stability of the composite under photocatalytic process.
Light-induced active species play an important role in the photocatalytic process for the rate of degradation, including the hole (h + ), superoxide anion radical (•O 2 − ), and hydroxyl radical (•OH).Scavengers were used for three main reasons: (1) postponing the electron/hole recombination formed on the surface of the photocatalyst, (2) trapping the electron/hole or radical and increasing the performance efficiency by removing the interference effects of active compounds in the environment and increasing the role of the effective agent in pollutant decomposition and (3) playing a direct role in facilitating the breaking of the color agent bonds in the dye molecule [30][31][32] .
In this regard, methanol acts as a hydroxyl radical scavenger, ethanol was investigated as an electron absorber and sodium chloride as h + Scavenger (Fig. 11a).With the addition of ethanol and sodium chloride, it can be seen that greater drop in the degradation efficiency was occurred, therefore, holes and electrons are considered as "effective primary" factors in the degradation mechanism.The results of PL analysis showed that the synthetic photocatalyst (ZIF-11/g-C 3 N 4 0.3) is suitable for the separation of charge carriers.In other hand, the results of the active species trapping experiments showed that hole and electron are effective factors in the degradation mechanism of MB.
The above topics were raised in order to further investigate the destruction mechanism.The charge carriers of ZIF-11/g-C 3 N 4 0. which have a more negative potential, reduce molecular oxygen and superoxide radical were formed to achieve efficiency.The holes in the valence band of ZIF-11, which have more positive potential, cause the creation of active hydroxyl radicals.As a result, it is a photocatalyst with a Z-scheme to produce reactive species •O 2 − and •OH for degradation of MB.The Z-scheme mechanism can summarize the charge transfer process and the ZIF-11/g-C 3 N 4 0.3 photocatalyst creates a more effective electron migration system that is directly involved in the photocatalytic degradation of organic dyes and ultimately increases the performance of the photocatalyst.
The comparison of current results with various studies was demonstrated in Table 6.It can be concluded that ZIF-11/g-C 3 N 4 0.3 which was synthesized by simple method at room temperature, has good performance in degradation of MB by using less power of irradiation and good stability under visible light irradiation.

Conclusion
In this research, the zeolitic-11 imidazolate framework/graphitic carbon nitride was synthesized for the first time by a simple method and its photocatalytic performance in the degradation of MB under visible light irradiation was investigated.The resulting structure leads to the formation of a Z-scheme photocatalyst, active in visible light with a lower electron/hole recombination rate than pure ZIF-11 in the degradation of MB.The higher surface area of ZIF-11/g-C 3 N 4 0.3 was comparable to pure g-C 3 N 4 .Also, the higher ability of light absorption by ZIF-11/g-C 3 N 4 0.3 indicated that g-C 3 N 4 in the structure plays a significant role in improving the photocatalytic activity.According to the results of the statistical analysis of the effect of operational parameters, two parameters of photocatalyst concentration and pH have more effective on photocatalytic degradation efficiency.The photocatalytic degradation of MB by ZIF-11/g-C 3 N 4 0.3 under optimal operating conditions follows pseudo first-order kinetic model.The photocatalytic degradation efficiency of MB under optimal operating conditions in three consecutive cycles by ZIF-11/g-C 3 N 4 0.3 did not have a significant efficiency loss, which indicates the reusability and acceptable stability of the synthetic photocatalyst.

Figure 1 .
Figure 1.(a) Schematic illustration of synthesis process of g-C 3 N 4 /ZIF-11 X, (b) schematic representation of the homemade photo-reactor.

Figure 7 .
Figure 7. 3D RSM (a) effect of photocatalyst concentration vs. time, (b) effect of pH vs. time (c) effect of pH vs. photocatalyst concentration.

Table 2 .
Specific surface area and pore characteristics of synthetic samples.

Table 3 .
The design matrix of the central complex; factors, levels, experimental responses.